Humans depend on acceptance into groups and intimate relationships for survival and emotional well- being. Actual or perceived threats to this need such as social rejection (when one is not wanted or liked) can lead to marked changes in mood and behavior such as sadness, social withdrawal, and impulsivity. The experience of severe or repeated social rejection in those who are rejection sensitive is a strong contributor to psychiatric disorders such as major depressive, social anxiety, and personality disorders. The neurotransmitter mechanisms underlying rejection sensitivity (RS) are not known. It has been known for over 30 years in nonhuman animals that the endogenous opioid system, particularly the -opioid receptor (MOR) system, regulates social distress and social reward behaviors. Using positron emission tomography, we recently showed that social rejection and acceptance produced robust MOR- mediated neurotransmission in specific brain areas, which correlated with changes in mood and behavior. This study was the first to show that the endogenous opioid system responds to social cues in humans. The proposed project will examine the MOR system in the clinically important trait of RS. Since the neurotransmitter mechanisms of RS are unknown, we seek to first understand the basic neurobiology of RS in a healthy population, prior to studying clinical populations. The overall hypothesis is that RS is associated with MOR function. Those with higher RS compared to lower RS are hypothesized to have overall lower MOR activation during social rejection and acceptance, leading to greater distress and dampened pro-social behavior. Numerous animal studies have also established that the MOR system is strongly influenced by harmful social environments. Therefore, we will also examine the role of childhood maltreatment (CM), a negative early life experience known to be one of the highest risk factors for developing depression and anxiety. The goal of this project is to determine how RS and CM interact to determine patterns of MOR binding during baseline, social rejection, and social acceptance in a healthy population. We will also examine how RS, mediated through MOR activation, influences mood and behavior. The impact of this research is to provide the first major step towards understanding a neurotransmitter mechanism for RS, with the long-term goal of predicting and treating its associated disorders.